Resonances in helium photoionisation

Abstract

The authors have developed an L² analogue of R-matrix theory to calculate the bound and scattering states of helium using large Sturmian basis sets. The first application is to the lowest several doubly excited ¹P resonances in the synchrotron photoionisation of the helium ground state. To facilitate comparison with experiment, the resonance shape parameters introduced by Fano were redefined in terms of Siegert boundary conditions to allow their ab initio calculation without fitting. Excellent agreement with recent highly precise experiments is obtained.